[CT-39] Write tests for the referral contract

[yusuftomilola]

Overview

Write a comprehensive test suite for the referral contract in src/test.rs. Work inside contracts/sandbox/referral/.

Acceptance Criteria

• Test: register_code creates code and rejects duplicates
• Test: use_referral_code links referee and increments usage count
• Test: use_referral_code rejects self-referral
• Test: use_referral_code rejects already-referred address
• Test: pay_referral_reward sends token to referrer and marks as paid
• Test: reward cannot be paid twice for the same referral
• All tests pass with cargo test

[xeeenab]

@xeeenab has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi, I’d like to work on implementing this feature and have reviewed the requirements. I’ll ensure it integrates cleanly with the existing codebase and follows the project’s guidelines, please let me know if there are any specific preferences before I begin.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @xeeenab to this issue.

[drips-wave]

Congratulations, @xeeenab! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @xeeenab: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[drips-wave]

This issue has been completed by @xeeenab as part of the Stellar Wave Program's 4th Wave 🥳

😎 @xeeenab: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.